1. Field of the Invention
The present invention relates to an optical disc apparatus, an optical disc method and a semiconductor integrated circuit capable of performing a recording or reproduction at a high density for an optical disc by accurately adjusting a focus position of a light beam and a spherical aberration amount.
2. Description of the Related Art
As a method for increasing the recording density for an optical disc, a method for reducing the size of a spot of a light beam formed on an information surface of the optical disc is known. The reduction of the size of the spot of the light beam formed on the information surface of the optical disc is achieved by increasing a numeral aperture (NA) of the light beam and decreasing a wavelength of the light beam.
However, when a numeral aperture (NA) of the light beam is increased and a wavelength of the light beam is decreased, a spherical aberration amount generated due to an error in the thickness of the protection layer of the optical disc is increased rapidly. Accordingly, it is required to provide means for correcting the spherical aberration amount.
FIG. 14a and FIG. 14b are views for explaining a spherical aberration.
FIG. 14a shows a state where the thickness from the surface of the optical disc 13 to the information surface 15 is optimal so that no spherical aberration amount is generated on the information surface 15.
A light beam emitted from a laser source is refracted by a protection layer 14 of the optical disc 13 in a state where the focus control is operated. As a result, an outer portion of the light beam is converged into a focus point B and an inner portion of the light beam is converged into a focus point C. A position A is located on a line connecting the focus point B and the focus point C, and is also located on the information surface 15. Since no spherical aberration amount is generated on the information surface 15 of the optical disc 13, both the focus point B of the outer portion of the light beam and the focus point C of the inner portion of the light beam conform with the position A. That is, a surface which has an equivalent distance from the position A conforms with a wave surface of the light beam.
FIG. 14b shows a state where the thickness from the surface of the optical disc 13 to the information surface 15 is insufficiently small and a spherical aberration amount is generated on the information surface 15.
Since the thickness from the-surface of the optical disc 13 to the information surface 15 (i.e. the thickness of the protection layer 14) is small, the influence of the spherical aberration amount is large. The focus point B and the focus point C are separated from each other. The two focus points B and C are in a defocus state with respect to the position A of the information surface 15 onto which the light beam is to be converged. The position A is located on the information layer 15 since the FE signal is generated without separating the outer portion of the light beam with the inner portion of the light beam and the focus control is operated such that the FE signal becomes almost zero. A wave surface of the light beam does not conform with a surface which has an equivalent distance from the position A.
In FIG. 14b, a solid line indicates the inner portion and the outer portion of the light beam in the state where the spherical aberration is generated, and a dotted line indicates the inner portion and the outer portion of the light beam in the state where the spherical aberration is not generated.
When the thickness as defined from the surface of the optical disc 13 to the information surface 15 is greater than the thickness shown in FIG. 14a, the focus point B and the focus point C are separated from each other, and the two focus points B and C are in a defocus state with respect to the position A of the information surface 15 onto which the light beam is to be converged in a similar way to the case shown in FIG. 14b. 
Thus, a phenomena in which the focus point B of the outer portion of the light beam and the focus point C of the inner portion of the light beam are separated from each other is referred to as a “spherical aberration”. The amount of the spherical aberration is referred to as a spherical aberration amount or a spherical aberration generation amount.
FIG. 8 shows a procedure of a conventional method for adjusting the focus position of the light beam and the spherical aberration amount, which is described in Japanese laid-open publication No. 2002-342952 (pages 4-6, FIG. 1).
A servo control including a focus control, a tracking control and a disc motor servo is started (ON) at step S31. Next, a multi-dimensional search routine is performed at step S32 in order to correct the focus position of the light beam and the spherical aberration amount. Under the control of the microcomputer, the focus position of the objective lens is wobbled in accordance with a focus disturbance signal. In parallel to this, a spherical aberration correction disturbance signal is supplied to the spherical aberration correction driving circuit. As a result, a spherical aberration correction amount is wobbled. Such a search is performed in a multi-dimensional space (two-dimensions and eight directions) as shown in FIG. 9, so that the focus position of the light beam and the spherical aberration correction amount are sequentially adjusted to increase an envelope signal of the RF signal.
However, during the process for adjusting the focus position of the light beam and the spherical aberration amount, there may be a case where the amplitude of the tracking error signal (TE signal) is reduced rapidly and the tracking control becomes unstable.
FIG. 10 shows the characteristic of an envelope signal of the RF signal with respect to the focus position of the light beam and the spherical aberration amount. In FIG. 10, the lateral axis indicates a focus position of the light beam and the vertical axis indicates a spherical aberration amount generated in a spot of the light beam formed on the information surface 15 of the optical disc 13. The value of the envelope signal is represented by a contour line map including a plurality of concentric ellipses. The value of the envelope signal on a contour line is constant. The value of the envelope signal becomes higher as the point on the map approaches the center of each ellipse. Accordingly, the value the envelope signal becomes maximum at approximately the center of each ellipse.
FIG. 2b shows the amplitude characteristic of the TE signal with respect to the focus position of the light beam and the spherical aberration amount. The lateral axis and the vertical axis shown in FIG. 2b are the same as those shown in FIG. 10. The amplitude level of the TE signal is represented by a contour line map including a plurality of concentric ellipses. The amplitude level of the TE signal on a contour line is constant. The amplitude level of the TE signal becomes higher as the point on the map approaches the center of each ellipse. Accordingly, the amplitude level of the TE signal becomes maximum at approximately the center of each ellipse.
According to the conventional method, during the process for adjusting the focus position of the light beam and the spherical aberration amount such that the envelop signal of the RF signal becomes maximum, there may be a case where the tracking control becomes unstable and the adjustment is terminated unsuccessfully.
For example, in a case where the search is started from a point A shown in FIG. 2b or the point approaches the point A during the search, the amplitude of the TE signal is extremely reduced, and the gain of the tracking control system is reduced. Accordingly, there may be a case where a control residual is increased or the tracking control system is oscillated.
One of the purposes of the present invention is to provide an optical disc apparatus, an optical disc method and a semiconductor integrated circuit capable of adjusting the focus position of the light beam and the spherical aberration amount such that the reproduced signal quality becomes optimal while maintaining the stability of the tracking control.